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I propose that be marked as a synonym of .

Checksums are not always hashes, for example parity bits on network packets, but I think that for security purposes, people use "checksum" as a layman's term for "hash", ex.: the three questions tagged with are all about checking the integrity of files.

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Replace the tag to fit the OP's intentions. Don't mark it as a synonym.

I do not think they should be merged. I think those three questions tagged as should simply be edited to use instead. There is a very big difference between checksums and hashes. A hash is a stateless function that maps data of an arbitrary size to a fixed size in the form of ƒ : {0,1}* → {0,1}n. A checksum on the other hand is typically nothing more than the weighted summation of a function of each data word (whether said function is a lookup table or the identity function), modulo a small value. This is a very specific definition and does not at all apply to hashes. If only three people are using the tags, just replace it, don't merge it.

I've went ahead and made the edits since this answer has been the most upvoted for the past 18 days. If I went ahead too soon, feel free to revert the edits for more discussion.

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    “A checksum (…) is nothing more than the weighted summation …” No, that's just one possible way to build a checksum. More generally, a checksum is a stateless function that maps data of arbitrary size to a fixed size. That's exactly the same formal definition as a hash. The definition comes from the purpose, and cryptographic hashes are actually practically-perfect checksums. – Gilles 'SO- stop being evil' Aug 14 '18 at 19:52
  • @Gilles Checksums use addition (hence the term sum) for their calculations in some manner. Hash algorithms may use any number of operations. – forest Aug 14 '18 at 20:49
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Indeed, people asking about checksums here do seem to want to use them for some security purpose, which means that they're looking for a cryptographic hash (or they're wrong about wanting a secure checksum). The tag hasn't been used for anything else and I can't think of an alternative meaning. Therefore should be made a synonym of and merged, to help people who don't know the correct name for what they're asking about.

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I would argue that they should be different. They are mathematically, and cryptographically different. They have different computational requirements, and more fundamentally they are used for different end goals.

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  • It's not about whether the words are different, but whether they way people use the words in the context of this site. I'm with Gilles that when someone says "I want to checksum my files to prevent tampering", what they really mean is cryptographic hash, so the tags should point them to the correct wording. Actual checksums are used, for example, at the TCP layer, but I'm not sure that has any security implications .. ? – Mike Ounsworth Aug 15 '18 at 13:42
  • @MikeOunsworth There are some circumstances where a true checksum is related to security. – forest Aug 15 '18 at 19:02
  • @forest Care to provide some examples? – Mike Ounsworth Aug 15 '18 at 20:30
  • @MikeOunsworth For example when used for integrity against data corruption (e.g. the CRC used over SATA). Corruption can have security implications even if it is not caused by an attacker. Additionally, checksums may be abused for security, like with ZipCrypto. The security of Zip file encryption is obviously on-topic, even though it is based on CRC and as a result is broken. Also, there are some cryptographic algorithms that may use non-cryptographic primitives at their core (e.g. E0 and A5/1 which uses LFSRs which are not security-related on their own). Or what about CRC in SSHv1? – forest Aug 15 '18 at 23:10
  • @forest Interesting, and valid. However I don't see any of those questions having even been asked on this site with the tag [checksum], so it's weighing people using it correctly vs people using it incorrectly. – Mike Ounsworth Aug 16 '18 at 0:02
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    @MikeOunsworth Indeed. As I mentioned in my answer, I think those specific examples should have their tags changed, not merged. – forest Aug 16 '18 at 0:04
  • Given that there are only 3 questions with the checksum tag, I do not think that there is a massive problem with people using the wrong tag, cant we simply change the tags on those questions? – Colin Cassidy Aug 17 '18 at 8:49
  • @ColinCassidy That's what I propose should be done. – forest Aug 17 '18 at 10:17

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